[LoopVectorize][NFC] Split reductions out from sve-tail-folding into new file

In sve-tail-folding-reductions.ll I've also added an extra RUN line
to test normal reductions, i.e. not in-loop. This patch is a pre-commit
in preparation for a follow-on patch that changes how reduction selects
are generated in the vector loop.

Differential Revision: https://reviews.llvm.org/D129761